Lost limbs are gone, but not forgotten by the brain. Two studies in this week's Science help explain why this memory persists. The research shows how the brain miswires itself after an amputation, connecting neurons, say from the face, to the brain region that once sensed an arm. Stopping or undoing the rewiring could help treat phantom limb pain, in which an appendage throbs even when it's missing.

Researchers knew that some signals get crossed after an amputation. To pinpoint where this happens, Edward Jones[2], a neuroscientist at University of California, Davis, studied eight monkeys whose arms had been amputated over the last 20 years. Jones inspected the thalamus, the last stop for sensory signals as they move from the spinal column out to the brain's cortex. He found that the neurons assigned to the arm were withering away, while nearby neurons devoted to the face had sprouted new dendrites to take up the available real estate.

The rewiring didn't stop there. In a second study, Sherre Florence[3], a psychologist at Vanderbilt University, looked at the brain's cortex, which processes signals from the thalamus. Her team took four monkeys that had lost arms in accidents and squirted dye onto neurons that once had received input from the arm. The dye showed that here, too, the arm neurons had linked up with face neurons up to several millimeters away.

The extent of the rewiring is a "substantial surprise," says Michael Merzenich, a neuroscientist at the University of California, San Francisco. Scientists hadn't thought remapping would occur in the thalamus, which is thought to be a simple relay station for nerve signals, he says. Both groups hope to find the chemical signals that instruct nerves to invade lost brain regions. "If we can control the reorganization, there is a potential cure for phantom limb pain," Merzenich says. Controlling nerve reorganization could also help stroke victims recover lost function, the teams speculate.